import java.util.Scanner;

public class frog {
    public static void main(String[] args) {
        Scanner scanner=new Scanner(System.in);
        int width=scanner.nextInt();
        int day=scanner.nextInt();
        int sum=day*2;
        int x;//代码需要的跳跃能力
        int []arr=new int[width-1];
        int i=0;
        int f=width;
        while (f>1){
            int high=scanner.nextInt();
            arr[i]=high;
            i++;
           f--;
        }
        int s=0;
        for(int j=0;j<arr.length;j++){
           s+=arr[j];
        }
        x=(width-1)/(s/sum);
        System.out.println(x);

    }
}
